The present invention relates in general to water heater construction concepts and methods. More particularly, the present invention relates to the construction of water heaters out of plastic with the annular insulation space between the tank and shell being evacuated.
Conventional commercial and residential water heater construction typically includes a metal inner storage tank and an outer metal appearance/protective shell. The tank is typically made from heavy gauge metal and the shell from lighter gauge sheet metal. The tank and shell are each substantially cylindrical and arranged concentric to each other. By sizing the shell larger than the outside diameter of the tank, an annular space is created between the tank and shell allowing insulation material to be placed in the annular space for thermal insulation around the tank. Insulating material may be either fiberglass or urethane foam or a combination of an insulation blanket and foam-in-place insulation. Examples of such metal construction and foam insulation are found in U.S. Pat. No. 4,736,509 issued Apr. 12, 1988 to Nelson, U.S. Pat. No. 4,372,028 issued Feb. 8, 1983 to Clark et al., U.S. Pat. No. 4,477,399 issued Oct. 16, 1984 to Tilton and U.S. Pat. No. 4,749,532 issued June 7, 1988 to Pfeffer.
While a variety of insulation designs and material combinations have been conceived and tried, each necessitates a number of process and assembly steps in the construction of a water heater. The metal construction of tank and shell also involved a number of processing steps and some complexity of handling due to the materials and weight. It would be an improvement to conventional water heater construction to reduce the number and complexity of manufacturing steps and to reduce the material and handling costs.
Another concern exists with conventional foam insulation which is frequently employed in the annular space between the tank and shell. Even when a blanket of fiberglass is used around the lower portion of the tank, liquid foam-in-place insulation is injected into the enclosed space above the blanket of fiberglass. The use and application of such foam-in-place insulation releases fluorocarbons into the atmosphere, similar to aerosol sprays. While such releases into the atmosphere reduce the ozone layer and are thus adverse environmentally, there is an equally significant concern regarding compliance with federal regulations. In the event regulations are passed which restrict the release of fluorocarbons to the atmosphere as part of product assembly and fabrication, the water heater industry will face significant fabrication problems. In order to deal with such potential regulations, a concept to reduce the foam-in-place insulation will be required.
One embodiment of the present invention deals with the adverse release of fluorocarbons by eliminating all foam-in-place insulation from the construction of water heaters. This is accomplished by constructing the tank and shell from a molded plastic and pulling a vacuum on the annular space between these two generally concentric members. Even if regulations are not imposed and foam insulation continues to be used, plastic construction has a number of advantages from the standpoint of fabrication and material efficiencies. Since most plastic materials have a high degree of weldability and formability, there are numerous sealing configurations which can be used to retain urethane foam. Further, plastic construction precludes corrosion concerns and reduces construction and labor costs. In view of the extensive efforts in prior devices to seal the annular space for receipt of urethane foam, the elimination of costly sealing devices and structures is a great advantage in water heater construction.
The following patent references each disclose plastic construction concepts associated with water heaters and while each may disclose certain concepts and features of interest relative to the present invention, none of the references listed below are believed to be anticipatory of the present invention nor would render the present invention obvious in combination with other references.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 2,089,950 (UK) Duncan 06/30/82 3,275,798 Martin 09/27/66 3,207,358 Fliss 09/21/65 2,718,583 Noland 09/20/55 DL0132031 (German) Forste 08/16/78 1,454,713 (German) Benkert 03/20/69 2,012,302 (German) Kaltenecker 03/14/70 2,911,880 (German) Burkardt 10/02/80 7507991 (French) 07/11/75 ______________________________________
The U.K. patent discloses a thermally insulated container, such as a domestic hot water heater, which includes a double-walled plastic tank sealed closed by a double-walled annular plastic end cover. A plurality of spacers are used between the two walls of the tank to support the inner wall from the weight of the water contained therein. The space between the two walls is evacuated to provide thermal insulation. Spacers are also used between the two walls of the end cover. An electric heating element is disposed in the lower volume of the tank and a float controls the entry of water into the tank via a cooperating water inlet pipe. There is also a water outlet pipe and an overflow pipe, all of which communicate with the interior of the tank by passing horizontally through the double side wall of the tank. This device is not pressurized and thus would not be suitable for use in the U.S. in view of the conventional water pressures which exist.
Martin discloses a domestic electric appliance configured as a water heater wherein the tank is defined by a double thickness of material including a generally cylindrical sheet metal or other suitable material for the outer wall and a layer of cellular plastic foam on the inside of the sheet metal shell. It is intended that the plastic foam have good insulating qualities and include some type of impervious skin on the innermost surface which will be the surface in direct contact with the water in the tank. This particular structure is not a plastic tank and plastic shell combination, nor is there any cavity to be evacuated.
Fliss discloses a water storage tank which is configured with an outer steel wall which is coated with a resin liner which includes an inner layer bonded to the outer wall and an outer layer which is bonded to the inner layer. Again, this particular device does not include a plastic construction and a double wall design where the annular space between the tank and the shell is evacuated.
Noland discloses the construction of a reinforced plastic water heater tank which is formed by helically winding sequentially-setting-plastic-impregnated glass-fiber "roving" cords over and around convex tank heads and helically around a cylindrical lining shell connected therebetween. This particular reference focuses specifically and primarily on the fabrication technique to simply construct a plastic hollow cylinder. Noland does not relate to plastic tank and shell designs for water heaters wherein the annular space between the shell and tank is evacuated.
The German patent of Forste discloses a hot water tank with a plastic inner liner. This plastic liner is clamped onto its flanged base by a metal adapter ring. The heating coil is housed inside a protective shell with a temperature sensor along side. The water supply pipe terminates inside the shell. This particular device does not include a double-walled construction wherein the space between the two walls is evacuated.
The German patent to Benkert discloses a hot water source with a sealed inner tank of a plastic foil and a foam insulating body which surrounds the inner tank. This particular structure does not include a double-wall configuration where the tank is plastic, the outer shell is plastic and the annular clearance space between the two is evacuated.
The German patent to Kaltenecker discloses a water heater which includes a blow-molded plastic tank with mountings which are configured as hollow mushroom-shaped keys which are engaged into a slightly smaller hole in a rear wall plate. While the primary focus of this particular reference is on this method of mounting, it is also important to note that the plastic tank is not a double-walled construction and there is no annular space present which could be evacuated in accordance with the present invention.
The German patent to Burkardt disloses a heat storage tank of rectangular cross section for the storage of liquid which is designed to work in conjunction with a solar heat collector system or a heat pump. The storage tank is made of plastic with a high temperature resistance and a low heat conductivity such as polypropylene or polyethylene. This storage liquid is not pressurized and its pressure is merely the static head. An input heat exchanger in the form of a coiled tube is arranged near the bottom with output heat exchanger near the top. Again, this particular design does not include the plastic construction of tank and shell with an evacuated annular clearance space therebetween.
The French patent discloses a storage type water heater which is heated electrically and which consists of a plastic cylinder with plastic end plates. The heater has a horizontal cylindrical body which is made of a plastic material and which is rigid and a poor conductor of heat. This particular structure does not include a double-walled construction with plastic for the tank and a plastic construction for the outer shell with an evacuated annular space therebetween.